[PATCH bpf-next v10 0/4] bpf: Add bpf_rcu_read_lock() support

Yonghong Song
2022-11-24T05:32:16Z

Currently, without rcu attribute info in BTF, the verifier treats
rcu tagged pointer as a normal pointer. This might be a problem
for sleepable program where rcu_read_lock()/unlock() is not available.
For example, for a sleepable fentry program, if rcu protected memory
access is interleaved with a sleepable helper/kfunc, it is possible
the memory access after the sleepable helper/kfunc might be invalid
since the object might have been freed then. Even without
a sleepable helper/kfunc, without rcu_read_lock() protection,
it is possible that the rcu protected object might be release
in the middle of bpf program execution which may cause incorrect
result.

To prevent above cases, enable btf_type_tag("rcu") attributes,
introduce new bpf_rcu_read_lock/unlock() kfuncs and add verifier support.

In the rest of patch set, Patch 1 enabled btf_type_tag for __rcu
attribute. Patche 2 added might_sleep in bpf_func_proto. Patch 3 added new
bpf_rcu_read_lock/unlock() kfuncs and verifier support.
Patch 4 added some tests for these two new kfuncs.

Changelogs:
v9 -> v10:
. if no rcu tag support in vmlinux btf, using bpf_rcu_read_lock/unlock()
will cause verification error.
. at bpf_rcu_read_unlock(), invalidate rcu ptr to PTR_UNTRUSTED
instead of SCALAR_VALUE.
. a few other comment changes and other minor changes.
v8 -> v9:
. remove sleepable prog check for ld_abs/ind checking in rcu read
lock region.
. fix a test failure with gcc-compiled kernel.
. a couple of other minor fixes.
v7 -> v8:
. add might_sleep in bpf_func_proto so we can easily identify whether
a helper is sleepable or not.
. do not enforce rcu rules for sleepable, e.g., rcu dereference must
be in a bpf_rcu_read_lock region. This is to keep old code working
fine.
. Mark 'b' in 'b = a->b' (b is tagged with __rcu) as MEM_RCU only if
'b = a->b' in rcu read region and 'a' is trusted. This adds safety
guarantee for 'b' inside the rcu read region.
v6 -> v7:
. rebase on top of bpf-next.
. remove the patch which enables sleepable program using
cgrp_local_storage map. This is orthogonal to this patch set
and will be addressed separately.
. mark the rcu pointer dereference result as UNTRUSTED if inside
a bpf_rcu_read_lock() region.
v5 -> v6:
. fix selftest prog miss_unlock which tested nested locking.
. add comments in selftest prog cgrp_succ to explain how to handle
nested memory access after rcu memory load.
v4 -> v5:
. add new test to aarch64 deny list.
v3 -> v4:
. fix selftest failures when built with gcc. gcc doesn't support
btf_type_tag yet and some tests relies on that. skip these
tests if vmlinux BTF does not have btf_type_tag("rcu").
v2 -> v3:
. went back to MEM_RCU approach with invalidate rcu ptr registers
at bpf_rcu_read_unlock() place.
. remove KF_RCU_LOCK/UNLOCK flag and compare btf_id at verification
time instead.
v1 -> v2:
. use kfunc instead of helper for bpf_rcu_read_lock/unlock.
. not use MEM_RCU bpf_type_flag, instead use active_rcu_lock
in reg state to identify rcu ptr's.
. Add more self tests.
. add new test to s390x deny list.

Yonghong Song (4):
compiler_types: Define __rcu as __attribute__((btf_type_tag("rcu")))
bpf: Introduce might_sleep field in bpf_func_proto
bpf: Add kfunc bpf_rcu_read_lock/unlock()
selftests/bpf: Add tests for bpf_rcu_read_lock()

include/linux/bpf.h | 4 +
include/linux/bpf_verifier.h | 5 +-
include/linux/compiler_types.h | 3 +-
kernel/bpf/bpf_lsm.c | 6 +-
kernel/bpf/btf.c | 3 +
kernel/bpf/helpers.c | 14 +
kernel/bpf/verifier.c | 167 ++++++++--
kernel/trace/bpf_trace.c | 4 +-
tools/testing/selftests/bpf/DENYLIST.aarch64 | 1 +
tools/testing/selftests/bpf/DENYLIST.s390x | 1 +
.../selftests/bpf/prog_tests/rcu_read_lock.c | 158 ++++++++++
.../selftests/bpf/progs/rcu_read_lock.c | 290 ++++++++++++++++++
12 files changed, 621 insertions(+), 35 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/rcu_read_lock.c
create mode 100644 tools/testing/selftests/bpf/progs/rcu_read_lock.c

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2.30.2